After completing the baseline questionnaire, subjects will receive proposed intervention according to their group assignment. Four consecutive follow-ups (at 1, 3, 6 and 12 months) will be conducted over the telephone by trained interviewers. Interviewers will first carry out outcome assessments with blinding to group status. Then, the group status will be disclosed so that the intervention group will receive the booster intervention and the control group will not. Smokers who have successfully quit smoking at 6-month will be invited to come back to the out-patient clinic and have biochemical validation tests (saliva cotinine test and exhaled CO test).